Integrated flare combustion control

ABSTRACT

A system for flare combustion control includes a sound speed measurement device for measuring sound speed in a flare vent gas, and a flare combustion controller including a memory and a processor. The processor is configured to receive the measured sound speed and determine, based on the measured sound speed, a molecular weight of the flare vent gas. The processor is further configured to determine, based on the determined molecular weight, a net heating value of the flare vent gas, and adjust the net heating value of the flare vent gas by regulating an amount of a supplemental fuel gas in the flare vent gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 62/355,744 filed Jun. 28, 2016, entitled“INTEGRATED FLARE COMBUSTION CONTROL SYSTEM AND METHODS”, the disclosureof which is hereby incorporated by reference in its entirety.

BACKGROUND

The field of the present disclosure relates generally to gas flareoperations and more specifically, to methods and systems for controllingthe operating parameters of a gas flare used in industrial plants toburn flammable gasses.

At least some known flare control systems use a gas chromatograph (GC)to determine a flare gas composition and calculate a BTU value of theflare gas. This method does not directly measure a heating value of theflare gas, but rather speciates the sample and determines the heatingvalue by summing the products of each components' heating value and themolecular fraction of each component. The GC, however, is not acontinuous measuring instrument and has a slow response time that cantake many minutes for a BTU determination for each gas sample. Inaddition, the data received from the GC is many minutes old, and, as iscommon in flare systems, the composition of a flare vent gas can besubject to fast fluctuations and variances.

At least some other known flare control systems use a calorimeter tocalculate a BTU value of the flare gas. A calorimeter mixes and burns asample of the flare gas with air or another fuel. At least some knowncalorimeters regulate the flow of air to maintain a constant exhausttemperature. The air flow variation provides an input that can be usedto calculate a heating value of the flare gas. Some other calorimetersmeasure excess oxygen content after combustion of the flare gas sample.The residual oxygen content provides an input that can be used tocalculate a heating value of the flare gas. Such calorimeters, however,can have a response time greater than one minute or more for a BTUdetermination for each gas sample. In addition, a calorimeter cannotspeciate the gas sample, and therefore, there is still a need to have aGC installed in order to determine a total hydrocarbon content and/or amolecular weight of the flare vent gas.

BRIEF DESCRIPTION

In one aspect, a flare combustion control system is provided. The flarecombustion control system includes a sound speed measurement device formeasuring sound speed in a flare vent gas. In addition, the controlsystem includes a flare combustion controller having a memory and aprocessor. The processor is configured to receive the measured soundspeed from the sound speed measurement device. In addition, theprocessor is configured to determine, based on the measured sound speed,a molecular weight of the flare vent gas. The processor is furtherconfigured to determine, based on the determined molecular weight, a netheating value of the flare vent gas, and adjust the net heating value ofthe flare vent gas by regulating an amount of a supplemental fuel gas inthe flare vent gas.

In another aspect, a method for maintaining a minimum net heating valuein a combustion zone of a flare tip is provided. The method includesdetermining a sound speed of a flare vent gas, and determining, based onthe determined sound speed, a net heating value of the flare vent gas.The method also includes adjusting the net heating value of the flarevent gas by regulating an amount of a supplemental fuel gas in the flarevent gas.

In another aspect, a method for smokeless combustion of a flare vent gasis provided. The method includes measuring a sound speed of the flarevent gas and determining, based on the measured sound speed, themolecular weight of the flare vent gas. The method also includescalculating, based on the determined molecular weight, an amount of anassist gas to achieve smokeless combustion of the flare vent gas, andregulating a flow of the assist gas for mixing with the flare vent gasto produce smokeless combustion of the flare vent gas.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic view of an exemplary industrial plant;

FIG. 2 is a flow chart of an exemplary variable initialization loop fordetermining a net heating value of a flare vent gas for a respectiveoperational loop of a flare combustion controller of FIG. 1;

FIG. 3 is a flow chart of an exemplary assist gas loop for determiningan assist gas ratio for a flare vent gas for a respective operationalloop of the flare combustion controller of FIG. 1;

FIG. 4 is a flow chart of an exemplary fuel loop for determining a flowrate of a fuel gas flow for a flare vent gas for a respectiveoperational loop of the flare combustion controller of FIG. 1;

FIG. 5 is a flow chart of an exemplary tip speed loop for determining atip speed of a flare vent gas for a respective operational loop of theflare combustion controller of FIG. 1; and

FIG. 6 is a block diagram of the flare combustion controller of FIG. 1.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of the disclosure. These features arebelieved to be applicable in a wide variety of systems comprising atleast one embodiment of the disclosure. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the embodimentsdisclosed herein.

DETAILED DESCRIPTION

In the following specification and the claims, reference will be made toa number of terms, which shall be defined to have the followingmeanings.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

Embodiments of the disclosure can provide techniques for optimizing theoperation of a flare stack in an industrial plant. Industrial plantssuch as oil platforms and refineries often use a gas flare to burn offexcess flammable gasses such as excess gas resulting from plant startup,shutdown, and emergency situations. Along with the excess gasses beingburned off, other elements such as fuel gas, steam, and/or air can beprovided to the flare stack to help control the burn. It can bedesirable to adjust the ratios of the various ingredients provided tothe flare using a control system to optimize the burning of the flare.In some embodiments disclosed herein, the control system includes anultrasonic measurement system for measuring the various ingredientsprovided to the flare such as the flare gas, steam, air and/or fuel gas.The ultrasonic measurement system can be used as part of a two-stage“coarse” and “fine” tuning operation of the flare stack. Otherembodiments are within the scope of the disclosure.

In particular, embodiments of the disclosure provide a control systemfor maintaining a minimum net heating value (NHV) in a combustion zoneof the flare, enhancing assist gas (e.g., steam) flow to the flare for asmokeless operation at the flare tip, and maintaining a limit on tipvelocity as a function of the NHV. Embodiments of the ultrasonicmeasurement system can provide an accurate and repeatable flowmeasurement of the flare gas derived from the sound speed of themeasured stream. The sound speed can be estimated using an ultrasonicflowmeter, although other techniques are possible. The sound speed, incombination with a temperature and pressure of the stream, can providean average molecular weight of an unknown hydrocarbon mixture. Thisinformation can be used by the control system to determine the netheating value, or British thermal unit (BTU) content of the unknownhydrocarbon mixture, which can be used for a coarse tuning of theoperation of the flare to provide for more efficient flare operation. Inaddition to the coarse BTU determination, the control system can alsoprovide a correlation between BTU content and steam and/or air flowrequirements to facilitate maintaining smokeless operation of the flare.

FIG. 1 is a schematic view of an exemplary industrial plant 10. As shownin the exemplary embodiment, industrial plant 10 can include a flarestack 12 configured for burning, or flaring, flammable gases dischargedby industrial plant 10. A flare gas (e.g. waste gas) source 14 can becoupled to flare stack 12 via a flare gas conduit 16 for introducing aflare gas flow 18 into flare stack 12. Industrial plant 10 can alsoinclude a fuel gas source 20 for supplementing flare gas flow 18. Fuelgas source 20 can be coupled to flare gas conduit 16 via a supplementalfuel gas conduit 22 for introducing a supplemental fuel gas flow 24 intoflare gas flow 18. In the exemplary embodiment, flare gas flow 18 or amixture of flare gas flow 18 and supplemental fuel gas flow 24 flowingthrough flare stack 12 is referred to as a flare vent gas, generallyindicated at 34.

As shown in the exemplary embodiment, an assist gas source 26 can alsobe coupled to flare stack 12 proximate an open end, or flare tip 32, offlare stack 12. Assist gas source 26, such as but not limited to a steamsource, can be coupled to flare stack 12 via an assist gas conduit 28for introducing an assist gas flow 30 to flare gas flow 18 and/orsupplemental fuel gas flow 24 proximate flare tip 32 to facilitatesmokeless flaring. In the exemplary embodiment, assist gas flow 30 is aflow of steam and/or air. Alternatively, assist gas 30 is asubstantially dry flow of air.

Furthermore, as shown in the exemplary embodiment, assist gas conduit 28can include an assist gas control valve 36 coupled in line with theconduit for controlling an amount of assist gas flow 30 to flare stack12. In addition, supplemental fuel gas conduit 22 can include asupplemental fuel gas control valve 38 coupled in line with the conduitfor controlling an amount of supplemental fuel gas flow 24 into flaregas flow 18. Supplemental fuel gas flow 24 may be provided to facilitateincreasing a heating value of flare gas flow 18 and for use as a purgegas to facilitate preventing air from entering flare stack 12 throughflare tip 32. Also, assist gas flow 30 may be provided to flare stack 12to facilitate smokeless flaring and reducing a heating value of flarevent gas 34 and to facilitate protecting flare tip 32 from overheatingand damaging the tip.

As shown in the exemplary embodiment, industrial plant 10 can alsoinclude a flare combustion control system 50. It should be noted thatflare combustion control system 50 described herein is not limited tothe particular flare control aspects described herein. One of ordinaryskill in the art will appreciate that flare combustion control system50, as described herein, may be used to control all operational aspectsof flare stack 12, including for example, flare-related systems ofindustrial plant 10, in any suitable configuration that enables flarecombustion control system 50 to operate as further described herein. Inthe exemplary embodiment, flare combustion control system 50 can befurther configured to maintain a minimum NHV in a combustion zone 40 offlare stack 12, enhance an amount of assist gas flow 30 to flare stack12 for smokeless operation at flare tip 32, and maintain a limit on tipvelocity as a function of the NHV. Flare combustion control system 50can include a flare combustion controller 52 coupled in communication toan industrial plant distributed control system (DCS) 62. DCS 62 includesat least one computing device 64 and a communication network 66 forcoupling to operable system devices (e.g., assist gas control valve 36,supplemental fuel gas control valve 38, and energy measuring device 60).Communication network 66 connections take many forms including, forexample, direct wired connections between each system device andcomputing device 64, for example, over a bus network, or system deviceswirelessly transmitting data to computing device 64. In the exemplaryembodiment, communication network 66 can be an interface for bothsending control commands to the individual system devices in industrialplant 10, and retrieving data from the system devices themselves (e.g.,assist gas control valve 36, supplemental fuel gas control valve 38, andenergy measuring device 60).

Furthermore, in the exemplary embodiment, DCS 62 can be coupled to anenergy measuring device 60 configured to measure one or more propertiesof flare vent gas 34. In an alternative embodiment, energy measuringdevice 60 can be coupled to flare combustion controller 52. In theexemplary embodiment, energy measuring device 60 can be configured tomeasure one or more specific characteristics of flare vent gas 34, forexample and without limitation, a NHV, a lower flammability limit, atotal hydrocarbon content (THC), and a molecular weight of flare ventgas 34. Energy measuring device 60 may be, for example, a gaschromatograph (GC), a calorimeter, or any other device that can measurethe specific characteristics of flare vent gas 34, as described herein.

In some embodiments, a GC can be configured to determine a compositionof flare vent gas 34 and calculate a BTU value of the flare gas. Todetermine a heating value of flare vent gas 34, the GC can speciate thesample and determine the BTU value by summing the products of eachcomponent's BTU value and the molecular fraction of each component. TheGC, however, is not a continuous measuring instrument and has a slowresponse time that can take as long as twenty minutes or more for a BTUdetermination for each gas sample. In other embodiments, a calorimetermay mix and burn a sample of flare vent gas 34 with air or another fuel.The calorimeter may regulate the flow of air to maintain a constantexhaust temperature. The airflow variation provides an input that can beused to calculate a heating value of the flare gas. Alternatively, thecalorimeter may measure excess oxygen content after combustion of theflare gas sample. The residual oxygen content can provide an input thatcan be used to calculate a heating value of the flare gas. Thecalorimeter, however, can have a response time greater than one minuteor more for a BTU determination for each gas sample. In addition, thecalorimeter cannot speciate the gas sample, and therefore, there isstill a need to have a GC installed in order to determine a THC contentand/or a molecular weight of flare vent gas 34. In addition, thecalorimeter may also require regular calibration and if the gasescomprising flare vent gas 34 vary, then the calorimeter would need to becalibrated with each of such gases.

As shown in the exemplary embodiment, DCS 62 can be coupled to a flarevent gas flowmeter 54, a supplemental fuel gas flowmeter 56, and anassist gas flowmeter 58. In addition, DCS 62 can be coupled incommunication to assist gas control valve 36 and supplemental fuel gascontrol valve 38 to facilitate adjusting assist gas flow 30 andsupplemental fuel gas flow 24, respectively. Adjusting assist gascontrol valve 36 and supplemental fuel gas control valve 38 canfacilitate controlling the operating parameters of a flare 42 of flarestack 12.

In the exemplary embodiment, flare vent gas flowmeter 54 can be a soundspeed measurement sensor system that performs a signal path measurementby directing sound waves (e.g., ultrasonic waves) through flare vent gas34, and processes the detected sound waves to determine sound speed andto derive the average molecular weight of an unknown hydrocarbon mixturepresent in flare vent gas 34. Flare vent gas flowmeter 54 may include,for example and without limitation, a time-of-flight sensor, a phasedifference sensor, and the like.

Flare combustion controller 52 can receive, via DCS 62, sound speed datafrom flare vent gas flowmeter 54, pressure, and temperature informationof flare vent gas 34, and can perform an iterative procedure to match ahypothetical composition or average molecular weight to the observedsound speed. Flare combustion controller 52 can store a number of tablesin which the critical pressure, compressibility, acentric factor, andheat capacity of hydrocarbon gas have been tabulated as functions ofmolecular weight. Flare combustion controller 52 can execute aprogrammed set of calculations for determining the sound speed of amixture of gases by using the virial equations, and can determine thevirial coefficients from these four properties of flare vent gas 34. Forexample and without limitation, flare combustion controller 52 canestimate the molecular weight of the hydrocarbon component of flare ventgas 34, and perform a sequence of estimates and calculations to resultin a predicted sound speed, which is checked against the measured soundspeed received from flare vent gas flowmeter 54. If different, flarecombustion controller 52 can set the next molecular weight estimate, andcheck its effect on predicted speed, until these converge to the averagemolecular weight of the hydrocarbon components present in flare vent gas34. Thus, flare vent gas flowmeter 54 can accurately determine theaverage molecular weight of component gases of interest present in flarevent gas 34 using only parameters such as temperature, pressure, andsound speed that are directly and readily measured by sound waveinterrogation or using simple sensors. In addition, usage of the virialequations with flare vent gas flowmeter 54 can yield the compressibilityfactor of flare vent gas 34. This parameter is important for determiningmass flow of flare vent gas 34.

In the exemplary embodiment, supplemental fuel gas flowmeter 56 andassist gas flowmeter 58 can also be sound speed measurement sensorsystems. Alternatively, supplemental fuel gas flowmeter 56 and assistgas flowmeter 58 can be any type of mass flow sensors that enableindustrial plant 10 to operate as described herein.

FIGS. 2-5 are flow charts of exemplary methods for controlling flowrates of fuel gas flow 24, assist gas flow 30, and flare vent gas 34 tofacilitate controlling the operating parameters of flare 42 of flarestack 12. The U.S. Environmental Protection Agency (EPA) has issuedregulations for the operating and monitoring requirements for flares atindustrial plants, for example, flare 42 of flare stack 12. The generalprovisions of the regulations specify that flares shall be operated atall times when emissions may be vented to them, and must be operatedwith no visible emissions except for periods not to exceed five minutesduring any consecutive two (2) hour period). In addition, theregulations specify that a flare must be operated with a minimum heatcontent requirement and tip velocity based on the type of flare. Theregulations include combustion zone operating limits for flares,including requirements to ensure that the combustible material presentis sufficient to ensure adequate combustion of the gas mixture. Thethree flare combustion zone operational limits include net heating value(Btu/scf), lower flammability limit, and total combustible fraction. Todemonstrate continuous compliance with each of the three operatinglimits discussed above, industrial plant 10, for example, needs tomonitor flow rate Q_(flare) of flare vent gas 34, flow rate Q_(assist)of assist gas flow 30, and specific characteristics of flare vent gas 34(e.g., heat content and composition). The regulations specify that theindustrial plant would be required to calculate 15-minute block data ona cumulative basis using the monitored parameters.

As discussed above, an industrial plant may use a calorimeter to monitorvent gas net heating value. This would limit, however, the plant todemonstrating compliance only with the net heating value operatinglimit. In addition, an industrial plant may use a device that monitorsthe total hydrocarbon content (THC) of the vent gas. However, use ofsuch a device would limit the plant to demonstrating compliance onlywith the combustibles concentration operating limit. To demonstratecompliance with all three operating limits, the EPA suggests that theuse of an on-line gas chromatograph (GC) will provide sufficientflexibility to determine compliance with all of the three combustionzone operating limits. However, as discussed above, the GC is not acontinuous measuring instrument and has a slow response time fordetermining a BTU determination for each gas sample. In industrialplants, e.g., industrial plant 10, the composition of flare vent gas 34can be subject to certain fast fluctuations and variances. When thegases present under these measurement conditions are used in bulkprocesses involving hydrocarbons, the monetary value of material flowingin the conduit can be substantial, and it is necessary to know itsvolume and composition with relatively high accuracy for processing,control, and environmental reasons, as described herein.

Flare combustion controller 52, as described herein, can provide a fullyintegrated flare combustion control system that can facilitatemaintaining operational compliance with all three operating limitsduring periods between the EPA required 15-minute intervals or othermeasurement intervals of energy measurement device 60 when thecomposition of flare vent gas 34 can be subject to fast fluctuations andvariances. As described herein, the BTU calculations of energymeasurement device 60 are referred to as fine tuning of the operation offlare 42 of flare stack 12, and the measurement intervals of energymeasurement device 60 are referred to as “fine tuning” intervals. Thesound speed measurements of flare vent gas flowmeter 54 and control ofthe operating limits of flare 42 based on the sound speed measurementsis referred to as coarse tuning of the operation of flare 42. Coarsetuning intervals are determined by a plant operator and can includeupdate periods of, for example and without limitation, 2 minutes, 1minute, 30 seconds, 15 seconds, and continuous updates.

FIG. 2 is a flow chart of an exemplary variable initialization loop 200for determining a net heating value (NHVvent) of flare vent gas 34 for arespective operational loop of the flare combustion controller 52. Inthe exemplary embodiment, flare combustion controller 52 can receive ameasured net heating value (NHVmeas) from energy measurement device 60(e.g., a GC) and/or a sound speed of the flare vent gas 34 from theflare vent gas flowmeter 54. The NHVmeas and the sound speed can beinput into variable initialization loop 200. Flare combustion controller52 can determine 202 whether the NHVmeas value was acquired inside theprevious cycle of flare combustion controller 52. If the NHVmeas valuewas acquired in the previous cycle 204, the flare combustion controller52 can set the NHVvent value equal to the NHVmeas value 206. Thisrepresents a fine tuning operation of the flare combustion controller52, i.e., the use of a BTU value determined by energy measuring device60.

In the exemplary embodiment, if flare combustion controller 52determines that the NHVmeas value was not acquired inside the previouscycle 208, flare combustion controller 52 can use the measured soundspeed of flare vent gas 34. This represents a coarse tuning of the flarecombustion controller 52, i.e., the use of a “coarse” BTU valuecalculated from the sound speed. The measured sound speed can becompared to the sound speed measured in the previous cycle of flarecombustion controller 52 to determine if the current sound speed fallswithin a predetermined tolerance range of the previous sound speed 210.If it does, then flare combustion controller 52 can set the NHVventvalue based on the measured sound speed 212. If, however, the measuredsound speed in not within the predetermined tolerance range 214, flarecombustion controller 52 can set the NHVvent value equal to an estimatedNHV (NHVest) 216. NHVest is determined by flare combustion controller 52from a data model that includes previously saved NHVmeas and sound speeddata. Flare combustion controller 52 can choose an NHVest value from thedata model that corresponds to the received NHVmeas and sound speeddata. The NHVvent value can then be then used by flare combustioncontroller 52 as an input for further control of flare stack 12.

FIG. 3 is a flow chart of an exemplary assist gas loop 300 fordetermining an assist gas ratio for flare vent gas 34 for a respectiveoperational loop of the flare combustion controller 52. In the exemplaryembodiment, flare combustion controller 52 can receive 302 inputs,including flow rate Q_(assist) of assist gas flow 30, flow rate Q_(fuel)of fuel gas flow 24, flow rate Q_(flare) of flare vent gas 34, assistgas valve position, fuel valve position, and NHVvent. Flare combustioncontroller 52 can determine 304 whether there is an established normalset point for an assist gas ratio. If no 306, then flare combustioncontroller 52 can establish 308 a set point and proceed to step 312. Forexample, the American Petroleum Institute (API) has a model tofacilitate determining a steam ratio for a flare based on severaloperating conditions. Steam can be used as assist gas flow 30 to promotecombustion of flare vent gas 34. Incomplete combustion may causeincandescence and eventually, black smoke due to solid carbon particles.Both are pollutants and should be avoided, where possible. Too muchassist gas can cause poor flare operation. Provided that Q_(flare) offlare vent gas 34 is measured, the required assist gas ratio can bemaintained throughout the required flare operating range.

In the exemplary embodiment, if the normal set point is established forthe assist gas ratio 310, flare combustion controller 52 can determinewhether the assist gas flow rate is achieving 312 the assist gas ratiobased on the input values described herein. If the assist gas flow isnot achieving 314 the assist gas ratio, flare combustion controller 52can determine whether the assist gas ratio is below the set point 316.If the assist gas ratio is below 318 the set point, flare combustioncontroller 52 can calculate an amount to open assist gas valve 36 (shownin FIG. 1) and either open 320 the valve 36 by that amount or alert theplant operator to open assist gas valve 36 by that amount. If the assistgas ratio is above 322 the set point, flare combustion controller 52 cancalculate an amount to close assist gas valve 36 and either close 324the valve 36 by that amount or alert the plant operator to close assistgas valve 36 by that amount. Flare combustion controller 52 then cancalculate the new assist gas ratio and return to step 312 to determinewhether the assist gas flow is achieving the desired assist gas ratio.

If flare combustion controller 52 determines that the assist gas flow isachieving 326 the assist gas ratio, the next step is to determinewhether there are visible emissions 328 at the flare stack 12. If thereare 330 visible emissions, flare combustion controller 52 can determinean increased set point 332 that facilitates elimination of the visibleemissions at flare stack 12. Flare combustion controller 52 then canreturn to step 312 to determine whether the assist gas flow is achievingthe desired assist gas ratio. If there are no 334 visible emissions,then flare combustion controller 52 has determined a satisfactory assistgas flow set point for the current set of input values.

In the exemplary embodiment, flare combustion controller 52 can be aself-training controller, i.e., flare combustion controller 52 can storedata regarding assist gas flow set points for the various input values,including the sound speed of flare vent gas 34. After it is determinedthat there are no 334 visible emissions, the plant operator can storethe current data regarding the satisfactory assist gas flow set pointfor the current set of input values in flare combustion controller 52.The data can include, for example, the flow rate Qflare of flare ventgas 34, the sound speed data, and the assist gas ratio. As data iscollected over time, flare combustion controller 52 can build an assistgas ratio model specific to flare stack 12 and industrial plant 10.Thus, flare combustion controller 52 can determine a set point for theassist gas ratio of flare stack 12 based on the input data, therebyforgoing the use of for example, the API steam ratio model. Furthermore,flare combustion controller 52 can use the data collected over time tofacilitate determining an assist gas ratio that prevents visibleemissions across a wide range of flare vent gas 34 compositions.Therefore, during periods between the EPA required 15-minute intervalsor measurement intervals of the GC when the composition of flare ventgas 34 can be subject to fast fluctuations and variances, flarecombustion controller 52 can facilitate maintaining emission freeoperation of flare stack 12 by monitoring and making coarse tuningadjustments to the assist gas flow rate Q_(assist) by opening and/orclosing assist gas valve 36, or alerting the plant operator to do thesame.

FIG. 4 is a flow chart of an exemplary fuel loop 400 for determining aflow rate Q_(fuel) of supplemental fuel gas flow 24 for flare vent gas34 for a respective operational loop of the flare combustion controller52. In the exemplary embodiment, flare combustion controller 52 candetermine a net heating value NHVcz of the combustion zone based atleast in part on the sound speed of flare vent gas 34 and the flow rateQ_(fuel). At 402, flare combustion controller 52 can determine whetherthe calculated NHVcz is less than the lower limit (e.g., set byregulation requirements and/or selected by a flare operator) for flare42 of flare stack 12. If yes 404, flare combustion controller 52 canopen 406 fuel valve 38 an amount or instruct the plant operator to dothe same. Flare combustion controller 52 can determine a new NHVcz basedon the increased flow rate Q_(fuel). If the NHVcz is not 408 less thanthe lower limit regulation requirements for flare 42 of flare stack 12,flare combustion controller 52 can determine 410 whether the NHVcz isgreater than an upper limit set in the regulation requirements.

In the exemplary embodiment, if the NHVcz is greater than 412 the upperlimit, flare combustion controller 52 can close 414 fuel valve 38 anamount or instruct the plant operator to do the same. Flare combustioncontroller 52 can determine a new NHVcz based on the decreased flow rateQ_(fuel). Steps 402 and 410 may be repeated by flare combustioncontroller 52 until it determines a steady state flow rate for Q_(fuel).If the NHVcz is not 416 greater than the upper limit for flare 42 offlare stack 12, flare combustion controller 52 has determined asatisfactory fuel flow rate Q_(fuel) for the current set of inputvalues.

FIG. 5 is a flow chart of an exemplary tip speed loop 500 fordetermining a tip speed of flare vent gas 34 for a respectiveoperational loop of the flare combustion controller 52. In the exemplaryembodiment, flare combustion controller 52 can determine a maximum tipspeed Vtip_max based on the NHVcz and can compare it to the actual tipspeed Vtip of the flare stack 12, at 502. If Vtip is less than Vtip_max524, flare combustion controller 52 can return to step 328 (also shownin FIG. 3) to determine whether there are any visible emissions emittingfrom flare stack 12. If Vtip is greater than Vtip_max 504, flarecombustion controller 52 can determine 506 whether NHVcz is greater thanthe lower limit set in the regulation requirements. If NHVcz is greaterthan 508 the lower limit, flare combustion controller 52 can close 510fuel valve 38 an amount or instruct the plant operator to do the same.

Flare combustion controller 52 can determine a new Vtip_max based on thechanged NHVcz and can compare it to the updated actual tip speed Vtip ofthe flare stack 12 at 502 and repeats the process. If, however, NHVcz isless than 512 the lower limit, flare combustion controller 52 can open514 fuel valve 38 an amount or instruct the plant operator to do thesame, thereby increasing NHVcz and Vtip_max. At 516, flare combustioncontroller 52 again can determine whether NHVcz is greater than thelower limit set in the regulation requirements. If NHVcz is still lessthan 518 the lower limit, flare combustion controller 52 can open 520fuel valve 38 an additional amount or instruct the plant operator to dothe same, thereby further increasing NHVcz and Vtip_max. If NHVcz isgreater than 522 the lower limit, flare combustion controller 52 canreturn to step 328 to determine whether there are any visible emissionsemitting from flare stack 12.

FIG. 6 is a block diagram of flare combustion controller 52 that may beused to perform monitoring and control of any piece of equipment,system, and process, such as, without limitation, monitoring, andprocessing of operating parameters of industrial plant 10. In theexemplary embodiment, flare combustion controller 52 can include amemory device 602 and a processor 604 that is coupled to memory device602. Processor 604 may include one or more processing units, such as,without limitation, a multi-core configuration. In some embodiments,executable instructions can be stored in memory device 602. Flarecombustion controller 52 can be configurable to perform one or moreoperations described herein by programming processor 604. For example,processor 604 may be programmed by encoding an operation as one or moreexecutable instructions and providing the executable instructions inmemory device 602. In the exemplary embodiment, memory device 602 can beone or more devices that enable storage and retrieval of informationsuch as executable instructions or other data. Memory device 602 mayinclude one or more computer readable media, such as, withoutlimitation, random access memory (RAM), dynamic RAM, static RAM, asolid-state disk, a hard disk, read-only memory (ROM), erasableprogrammable ROM, electrically erasable programmable ROM, ornon-volatile RAM memory. The above memory types are exemplary only, andare thus not limiting as to the types of memory usable for storage of acomputer program.

As used herein, the term “computer” and related terms, such as,“computing device”, are not limited to integrated circuits referred toin the art as a computer, but rather broadly refers to amicrocontroller, a microcomputer, a programmable logic controller (PLC),an application specific integrated circuit, and other programmablecircuits, and these terms are used interchangeably herein. Furthermore,as used herein, the term “real-time” refers to at least one of the timeof occurrence of the associated events, the time of measurement andcollection of predetermined data, the time to process the data, and thetime of a system response to the events and the environment.

Memory device 602 may be configured to store operational parametersincluding, without limitation, real-time and historical operationalparameter values, or any other type of data. In some embodiments,processor 604 can remove or “purge” data from memory device 602 based onthe age of the data. For example, processor 604 may overwrite previouslyrecorded and stored data associated with a subsequent time or event. Inaddition, or alternatively, processor 604 may remove data that exceeds apredetermined time interval. In addition, memory device 602 can include,without limitation, sufficient data, algorithms, and commands tofacilitate monitoring and processing sensor measurements received fromsensors coupled to an industrial plant flare system including, withoutlimitation, flare vent gas flowmeter 54, supplemental fuel gas flowmeter56, and assist gas flowmeter 58.

In some embodiments, flare combustion controller 52 can include apresentation interface 606 coupled to processor 604. Presentationinterface 606 can present information, such as a user interface, to auser 608. In one embodiment, presentation interface 606 can include adisplay adapter (not shown) that is coupled to a display device (notshown), such as a cathode ray tube (CRT), a liquid crystal display(LCD), an organic LED (OLED) display, or an “electronic ink” display. Insome embodiments, presentation interface 606 can include one or moredisplay devices. In addition, or alternatively, presentation interface606 can include an audio output device (not shown), for example, withoutlimitation, an audio adapter, a speaker, or a printer (not shown).

In some embodiments, flare combustion controller 52 can include a userinput interface 610. In the exemplary embodiment, user input interface610 can be coupled to processor 604 and receive input from user 608.User input interface 610 may include, for example, without limitation, akeyboard, a pointing device, a mouse, a stylus, a touch sensitive panel,such as, without limitation, a touch pad or a touch screen, and/or anaudio input interface, such as, without limitation, a microphone. Asingle component, such as a touch screen, may function as both a displaydevice of presentation interface 606 and user input interface 610.

In the exemplary embodiment, a communication interface 612 can becoupled to processor 604 and can be configured to be coupled incommunication with one or more other devices, such as DCS 62, and toperform input and output operations with respect to such devices whileperforming as an input channel. For example, communication interface 612may include, without limitation, a wired network adapter, a wirelessnetwork adapter, a mobile telecommunications adapter, a serialcommunication adapter, or a parallel communication adapter.Communication interface 612 may receive a data signal from or transmit adata signal to one or more remote devices, such as flare vent gasflowmeter 54, supplemental fuel gas flowmeter 56, and assist gasflowmeter 58.

Presentation interface 606 and communication interface 612 can both becapable of providing information suitable for use with the methodsdescribed herein, such as, providing information to user 608 orprocessor 604. Accordingly, presentation interface 606 and communicationinterface 612 may be referred to as output devices. Similarly, userinput interface 610 and communication interface 612 can be capable ofreceiving information suitable for use with the methods described hereinand may be referred to as input devices.

In the various embodiments of the present disclosure, portions of theprocessing operations performed by flare combustion controller 52 can beimplemented in the form of an entirely hardware embodiment, an entirelysoftware embodiment, or an embodiment containing both hardware andsoftware elements. In one embodiment, the processing functions performedby flare combustion controller 52 may be implemented in software, whichincludes but is not limited to firmware, resident software, microcode,etc.

Furthermore, the processing functions performed by flare combustioncontroller 52 can take the form of a computer program product accessiblefrom a computer-usable or computer-readable medium providing programcode for use by or in connection with a computer or any instructionexecution system (e.g., processing units). For the purposes of thisdescription, a computer-usable or computer-readable medium can be anycomputer readable storage medium that can contain or store the programfor use by or in connection with the computer or instruction executionsystem.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor or controller, suchas a general purpose central processing unit (CPU), a graphicsprocessing unit (GPU), a microcontroller, a reduced instruction setcomputer (RISC) processor, an application specific integrated circuit(ASIC), a programmable logic circuit (PLC), and/or any other circuit orprocessor capable of executing the functions described herein. Themethods described herein may be encoded as executable instructionsembodied in a computer readable medium, including, without limitation, astorage device, and/or a memory device. Such instructions, when executedby a processor, cause the processor to perform at least a portion of themethods described herein. The above examples are exemplary only, andthus are not intended to limit in any way the definition and/or meaningof the term processor.

The systems and methods described herein can facilitate controlling theoperating parameters of a gas flare, or flare, used in industrial plantsto burn flammable gasses. Specifically, the sound speed measurementsystem can provide an accurate and repeatable flow measurement of theflare gas derived from the sound speed of the measured stream. The soundspeed is calculated by a sound speed flowmeter. The sound speed, incombination with a temperature and pressure of the stream, can providean average molecular weight of the stream. This information can be usedby the control system to determine the BTU content of the stream, whichcan be used for a coarse tuning of the operation of the flare to providefor more efficient flare operation within the periods between themeasurement intervals of the GC when the composition of flare vent gascan be subject to fast fluctuations and variances.

The methods and systems described herein are not limited to the specificembodiments described herein. For example, components of each systemand/or steps of each method may be utilized independently and separatelyfrom other components and/or steps described herein. For example, themethod and systems may also be used in combination with other industrialsystems, and are not limited to practice only with the flare gas systemsdescribed herein. Rather, the exemplary embodiment can be implementedand utilized in connection with many other industrial applications.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, this is for convenienceonly. In accordance with the principles of the disclosure, any featureof a drawing may be referenced and/or claimed in combination with anyfeature of any other drawing.

This written description uses examples to disclose the systems andmethods described herein, including the best mode, and also to enableany person skilled in the art to practice the disclosure, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the disclosure is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims if they have structural elements that do not differ from theliteral language of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal language of theclaims.

While the disclosure has been described in terms of various specificembodiments, those skilled in the art will recognize that the disclosurecan be practiced with modification within the spirit and scope of theclaims.

What is claimed is:
 1. A flare combustion control system comprising: asound speed measurement device for measuring sound speed in a flare ventgas; and a flare combustion controller comprising a memory and aprocessor, wherein said processor is configured to: receive the measuredsound speed from the sound speed measurement device; determine, based onthe measured sound speed, a molecular weight of the flare vent gas;determine, based on the determined molecular weight, a net heating valueof the flare vent gas; and adjust the net heating value of the flarevent gas by regulating an amount of a supplemental fuel gas in the flarevent gas.
 2. The flare combustion control system of claim 1, whereinsaid processor is further configured to determine the molecular weightof the flare vent gas by iteratively estimating the molecular weight,predicting a sound speed based on the estimated molecular weight, andcomparing the predicted sound speed against the measured sound speed. 3.The flare combustion control system of claim 2, wherein said processoris further configured to predict the sound speed based on the estimatedmolecular weight using virial equations.
 4. The flare combustion controlsystem of claim 1, wherein the net heating value determined based on thedetermined molecular weight is a coarse net heating value determined atcoarse tuning intervals, said combustion control system furthercomprising an energy measurement device configured to determine, from asample of the flare vent gas, a fine net heating value at fine tuningintervals, wherein the fine tuning intervals are longer than the coarsetuning intervals.
 5. The flare combustion control system of claim 4,wherein the coarse tuning intervals include intervals of at least one of2 minutes, 1 minute, 30 seconds, 15 seconds, and continuous.
 6. Theflare combustion control system of claim 1, wherein said processor isfurther configured to: calculate, based on the determined molecularweight, an amount of an assist gas to achieve smokeless combustion ofthe flare vent gas; and regulate a flow of the assist gas for mixingwith the flare vent gas to produce smokeless combustion of the flarevent gas.
 7. The flare combustion control system of claim 6, whereinsaid processor is further configured to regulate the flow of the assistgas by regulating a flow of steam from a steam source.
 8. A method formaintaining a minimum net heating value in a combustion zone of a flaretip, said method comprising: measuring a sound speed of a flare ventgas; determining, based on the measured sound speed, a molecular weightof the flare vent gas; determining, based on the determined molecularweight, a net heating value of the flare vent gas; and adjusting the netheating value of the flare vent gas by regulating an amount of asupplemental fuel gas in the flare vent gas.
 9. The method of claim 8,wherein said determining the molecular weight of the flare vent gascomprises iteratively estimating the molecular weight, predicting asound speed based on the estimated molecular weight, and comparing thepredicted sound speed against the measured sound speed.
 10. The methodof claim 9, wherein said predicting the sound speed comprises usingvirial equations based on the estimated molecular weight.
 11. The methodof claim 8, wherein said determining the net heating value based on thedetermined molecular weight comprises determining a coarse net heatingvalue at coarse tuning intervals, said method further comprisingdetermining, from a sample of the flare vent gas using an energymeasurement device, a fine net heating value at fine tuning intervals,wherein the fine tuning intervals are longer than the coarse tuningintervals.
 12. The method of claim 11, wherein said determining thecoarse net heating value at coarse tuning intervals comprisesdetermining the coarse net heating value at coarse tuning intervals ofat least one of 2 minutes, 1 minute, 30 seconds, 15 seconds, andcontinuous.
 13. The method of claim 8, further comprising: calculating,based on the determined molecular weight, an amount of an assist gas toachieve smokeless combustion of the flare vent gas; and regulating aflow of the assist gas for mixing with the flare vent gas to producesmokeless combustion of the flare vent gas.
 14. The method of claim 13,wherein said regulating the flow of the assist gas comprises regulatinga flow of steam from a steam source.
 15. A method for smokelesscombustion of a flare vent gas, said method comprising: measuring asound speed of the flare vent gas; determining, based on the measuredsound speed, a molecular weight of the flare vent gas; calculating,based on the determined molecular weight, an amount of an assist gas toachieve smokeless combustion of the flare vent gas; and regulating aflow of the assist gas for mixing with the flare vent gas to producesmokeless combustion of the flare vent gas.
 16. The method of claim 15,wherein said regulating the flow of the assist gas comprises regulatinga flow of steam from a steam source.
 17. The method of claim 15, whereinsaid calculating, based on the determined molecular weight, the amountof the assist gas comprises: determining, based on the determinedmolecular weight, a net heating value of the flare vent gas; anddetermining, based on the determined net heating value, an assist gasratio corresponding to smokeless combustion.
 18. The method of claim 17,further comprising: determining that visible emissions are occurringfrom combustion of the flare vent gas; and increasing the assist gasratio.
 19. The method of claim 18, further comprising: determining thatno visible emissions are occurring from combustion of the flare vent gasmixed with the flow of the assist gas at the increased assist gas ratio;and storing the increased assist gas ratio and the determined netheating value associated together in an operating model for smokelesscombustion.
 20. The method of claim 17, further comprising adjusting thenet heating value of the flare vent gas by regulating an amount of asupplemental fuel gas in the flare vent gas.